Posted
by
Soulskill
on Monday April 18, 2011 @06:50PM
from the attempting-the-full-isaac dept.

NotSanguine writes with this excerpt from the BBC:
"The neutrons are shot between two parallel plates, one above another and separated by about 25 micrometres — half a hair's width. The upper plate absorbs neutrons, and the lower plate reflects them. As they pass through, they trace out an arc, just like a thrown ball falling due to gravity. ... The new work by the ILL team has added what is known as a piezoelectric resonator to the bottom plate; its purpose is to jiggle the bottom plate at a very particular frequency. The researchers found that as they changed the bottom plate's vibration frequency, there were distinct dips in the number of neutrons detected outside the plates — particular, well-spaced 'resonant' frequencies that the neutrons were inclined to absorb. These frequencies, then, are the gravitational quantum states of neutrons, essentially having energy bounced into them by the bottom plate, and the researchers were able for the first time to force the neutrons from one quantum state to another. The differences in the frequencies — which are proportional to energy — of each of these transitions will be an incredibly sensitive test of gravity at the microscopic scale."

Wow, my physics courses apparently forgot to mention that Newton's Law of Gravity had anything to say about the quantum states of neutrons. In fact, I was taught it's not a law; it's a falsified hypothesis.

Wow, my physics courses apparently forgot to mention that Newton's Law of Gravity had anything to say about the quantum states of neutrons. In fact, I was taught it's not a law; it's a falsified hypothesis.

Newton's Law of Gravity can be seen as an approximation of Einstein's theory. We have to be careful when we speak of "falsified". We haven't discovered that gravity is proportional to 1/r, or that gravity isn't attractive but repulsive. We have discovered that Einstein's models are better predictors of experimental results. We can still us Newton's models to send humans to the Moon. But Newton's model makes no sense when asking questions such as "what would happen to the Earth if the Sun suddenly disappeared. It doesn't predict the bending of light, nor does it properly describe certain orbital phenomenon.

You don't need to know what the mass of light is, you only need to treat it as a classical particle traveling at 3e8 m/s. From classical mechanics, objects follow the same trajectory in a gravitational field regardless of mass (different orbits depend only on different initial positions/velocities); a beam of light can be treated just like a very fast moving comet. The mass of the light would only be important if you were trying to calculate the reverse effect of how much a passing light beam would move a planet/star as it passed. The fact that light travels at a finite speed has been known for a long time.

Zeroes are troublesome in physics. If a light particle has zero mass, it would mean that any force at all would cause an infinite acceleration. Since it was known that light travelled at finite speeds, it's reasonable to assume that light particles did have a mass (of course, classically there's no such thing as rest mass)

Two things. First: Newton knew that all objects (at least, all objects we've tested) accelerate toward the earth with a certain acceleration regardless of mass, depending only on their distance from the earth. If the law of gravitation is universal, then why wouldn't light also experience the same acceleration? Assuming that massless particles are an exception goes against Occam's Razor. Only if we observe that light does not deflect would we conclude our theory was wrong. Newton was unable to perform this experiment.

Second: Why would Newton automatically assume that light did not have mass? It seems perfectly obvious today, but is it obvious because everyone knows it or because it's obvious? I don't think it's obvious.

Two things: First the question is what deflection does Newton's law of gravity predict for the deflection of light, not did Newton know that light was massless.

Two: Newton and Cavendish did not know that light was massless but we do and so Newton's law predicts no deflection i.e. Cavendish got it wrong but so would anyone else predicting the behaviour of a particle which was discovered about 100 years after he wrote his paper!

Wrong. Photons have energy because they have momentum, they have zero mass...and since mass is a Lorentz invariant that is always true in all inertial frames. Einstein's proper energy-momentum relationship is:

E^2 = p^2 c^2 + m^2 c^4

So next time you see someone with an "E=mc^2" T-shirt on when they are moving you can point out that their T-shirt is wrong!

I think if Newton thought photons had 0 mass he'd have found it impossible to apply GMm/r^2 to them without getting the answer f=0 and deducing that photons did not curve in a gravity field.

But, what I failed to remember yesterday, is that Newton knew that light was particles. "How do I know that light is corpuscular? Because I grind my own lenses," or words to that effect. One of my favorite sayings, but apparently I only call it up when messing around with optics, not for gravity. He reasoned that sinc

I think if Newton thought photons had 0 mass he'd have found it impossible to apply GMm/r^2 to them without getting the answer f=0 and deducing that photons did not curve in a gravity field.

Remember also that Newton knew calculus, having invented much of it, which means he could have thought about it in terms of limits. F = ma = GMm/r^2. Take the limit of both sides as m goes to zero, and you get a = GM/r^2. No problem there. The burden is actually to come up with a reason why light wouldn't behave the sa

Gah. I didn't say that the right way, sorry. Intuitively, what I mean is that as m gets ever smaller and smaller the equality ma = GMm/r^2 continues to hold. In the limit as m goes to zero, both sides go to zero together, but that doesn't mean the acceleration must go to zero. I didn't state it right, I apologize.

Actually it does, but by half the amount predicted by general relativity.

Actually it does NOT. Cavendish made suppositions without any clue to what light was - the date on Cavendish's paper was 1804, well before even Maxwell's equations let alone photons were known. Having no clue about light they supposed that it would follow a trajectory as any massive body does - quite reasonable not given any evidence to the contrary - but now we know better!

Newton's statement of his law explicitly states that the attraction is proportional to the masses of the bodies and, since a photon

Firstly you are wrong in saying that a photon has no mass. It has no rest mass, but it certainly has energy, and even momentum (hv/c). Force is not just m*a, but even more correctly described as the change of momentum dp/dt. Gravity is a force and therefore it causes a change of momentum.There are many papers that speculate that is light is a particular, what must be the effect of gravity upon it, dating from several years before Cavendish even.

Firstly you are wrong in saying that a photon has no mass. It has no rest mass, but it certainly has energy, and even momentum (hv/c).

Wrong. I am completely and absolutely correct to say that the photon has no mass. Mass is something called a Lorentz invariant and does not change - it is a common mistake which people often make. The gamma in p=gamma*m*v comes from the velocity (look up 4-velocity) and not the mass. Hence a photon has zero mass in all inertial reference frames regardless of its energy. Even Einstein warned that it was wrong to think of the mass as changing.

Force is not just m*a, but even more correctly described as the change of momentum dp/dt.

The more you read the works of the ancient physicists, you will discover that they certainly were not people "without any clue", but often trying to wrap their minds around some serious observations.

If you use hindsight to hand pick the wild speculations which happened to hit near the mark and ignore the rest then of course it seems like they had some amazing insights. For example, Newton was an amazing physicist and mathematician, he even speculated that light was made of particles. He was also an alchemist and believed that he could transmute lead into gold.

The more you read ALL the works the ancient physicists, and not the works which have been selected with modern hindsight, the more you will r

I agree with the basic thrust of your post, but have one nitpick to make:

But Newton's model makes no sense when asking questions such as "what would happen to the Earth if the Sun suddenly disappeared.

Actually, general relativity doesn't answer this either, because GR has local conservation of mass-energy, so it doesn't allow the sun to disappear. A better example would be "What would happen to the Earth if the Sun suddenly zoomed away from us at nearly the speed of light." Admittedly I'm being totally pedantic here.

According to GR, the change in the gravitational field would propagate at the speed of light, so we wouldn't get any change in the gravitational field until 8 minutes later -- at the same time that the change became visibly detectable.

Well to be fair, it's a thought experiment. It's not saying 'is it possible for the sun to vanish', it's saying, 'IF the sun DID vanish, what would we experience from Earth'? Not that I'm against your pedantry per se of course;)

What are you basing this on? Has it been demonstrated? I personally haven't heard of gravity ever being repulsive. If you're talking about gravitational forces 'ejecting' objects that come close to massive bodies on certain trajectories then it's a bit of a stretch to say you're talking about a repulsive force.

But Newton's model makes no sense when asking questions such as "what would happen to the Earth if the Sun suddenly disappeared. It doesn't predict the bending of light, nor does it properly describe certain orbital phenomenon.

Actually, Newton's Gravitational laws _DID_ predict the bending of light by the Sun, but by a different amount!

There is a factor of 2 difference (can't remember which predicted the greater bending!).

For around 8 minutes nothing at all. Then we'd see the sun disappear, and we'd continue on a tangent from where we currently are in our orbit. At least that's my layman's understanding of things. Newton didn't appreciate that gravitational forces are not instantaneous, which is why you often hear the 'disappearing sun' thought experiment invoked when illustrating the differences between Newton and Relativity.

Under Einstein's Theory of Relativity, nothing can travel faster than light. If the Earth "disappeared", or some reasonable facsimile, it would take 7 - 8 minutes before we stopped receiving sunlight, since that is the time for light to travel from the Sun to the Earth. The Sun's gravity is what keeps pulling the Earth in a circular path...without it the Earth would travel in a fairly "straight line". If the Sun suddenly disappeared, its gravitational influence would also disappear. But Einstein's theor

This is news to me too. In the BBC article [bbc.co.uk], it says gravitational quantum states were only measured in 2002, in the parent experiment (the one where they didn't use piezo resonators -- just parallel plates).

If you bother to read the article, you see that they are trying to see whether good old Newtonian gravity is a good approximation at extremely small length and mass scales (scales where the additional accuracy provided by general relativity is unnecessary). They're trying to see if when you make the experiment this sensitive, do you see some kind of quantum effect. The answer so far seems to be no. Yes, the neutrons behave in a quantum mechanical way. The question is, do they behave as you'd predict if Newton's/Einstein's gravity is true, or do they do something unexpected? This has nothing to do with Newton vs. Einstein.

I disagree with the claim that it is falsified. All theories in physics come with two sets of conditions: the bounds in magnitude and the bounds in resolution. Newton's theory came with well-defined bounds - those of classical phenomena. You cannot extrapolate beyond those bounds and claim you are still working with the theory because the theory isn't defined beyond those bounds. Nor can you interpolate to the quantum level for the same reason - the theory isn't defined there.

Relativity didn't replace Newton's theory, it supplemented it. In computing terms, it's a third-party module you can add on. When you install the Relativity dynamic library, the combined theory applies to a much larger range of phenomena.

The only time a theory will actually be falsified is if QM's gravity or relativity can be patched to work within the other's range. They are contradictory and you cannot load both modules into Newtonian physics at the same time. Only one of these two will stand the test of time, the other will die. Whichever one wins will then merge with Newtonian mechanics to produce a universal law of gravity.

Newton's theory came with well-defined bounds - those of classical phenomena.

I'm not sure that's correct. In Newton's time the word "classical" in the sense of either non-relativistic or non-quantum didn't exist. Gravity and the other mechanical forces are continuous and space is three-dimensional with independent time dimension in Newton's model. The entire idea of "bounds" in terms of relative speed and size where Newtonian mechanics don't apply was invented much later (both pretty much by Einstein) and would likely have seemed ridiculous to Newton.

The idea of physical theories being bounded was actually quite popular in Newton's time - hence the phrasing of Hooke's Law, the approximation of the motion of pendulums, etc. The laws were all quite specifically written in the form "provided these preconditions are true, this result WILL apply". Newton didn't change this in the slightest.

Indeed, that should be obvious from the description of gravitational attraction. Object X is pulled towards object Y because of the mass of object Y alone. However, it is

What do you mean by the "addition law of vectors"? I've also never heard that Newton conceived universal gravitation as not necessarily universal and only applying in some limits. Newton actually said of Hooke's hypotheses on gravity that "without my Demonstrations, to which Mr Hook is yet a stranger, it cannot be beleived by a judicious Philosopher to be any where accurate"--Hooke's claims weren't sufficiently tested, either physically or mathematically, though Newton put his own similar/equivalent theory

The addition law of vectors, as stated by Gallileo, is that the relative vector of two bodies is given by the sum of the vectors for each body. In other words, take the components in the X, Y and Z directions for the velocity of each of the two, then the relative velocity vector is (X1+X2, Y1+Y2, Z1+Z2).

The planets are a very different mass than the sun and there's no suggestion in his model of planetary motion that he considered the sun plus planets to be orbiting a shared center of gravity. Rather, he was

I believe he did earth-moon calculations, and universal gravitation predicted an otherwise unobserved planet (Neptune?). Maybe Newton, in his calculations involving the sun, did always assume the sun was so much more massive than other planets that it was alright to assume it was at the center of mass of the solar system. That's a practical consideration that's different from what I understand you were saying in your first post, which was that Newton did not even theorize universal gravitation as applying t

I think it's fair to consider the relativistic velocity* of the particles as being the key part of the experiment for ATLAS (much as it was for EUROGAM, the one I worked on). However, even if you consider the relativistic velocity of the detectors, it would be relative to the byproducts of the collision and not to the observing scientist. Thus, the velocity of the experiment is still not zero -- except in summation. (Although the relativistic velocity is very high, the resultant velocity is nearly zero.)

This is a terrible apology for Newton's theory of gravity. Einstein's relativity isn't an "add on" module. It completely subsumes Newton's theory, and shows that is just a very good approximation at ordinary scales.

This in no way diminishes Newton's accomplishment, or even usefulness. However, we can say that his theory of gravity has been falsified.

There's a big difference between today's conception of Newtonian mechanics and (what I presume was) the original conception of it. Today, we call it an approximation; originally, it was perfectly precise. The original idea has been falsified, but the newer version with supplemental error bounds is alive and well. The add on analogy is a little bit of a stretch, but I don't think it's awful. Loading the GR module would be akin to adding routines that could predict the impact of a black hole meandering throug

Perhaps a kernel upgrade would be a better analogy for slashdot? The core of the system has had a major improvement but the desktop GUI is not really affected. So, unless you are a kernel hacker/gravitational physicist there are not many noticeable changes!

I'm not sure that you can say that it replaces the theory. You can use Newtonian mechanics exactly as Newton wrote them, without changing them one iota, but merely pass in the relativistic values rather than the classical ones. That doesn't sound like a replacement, that's a pre-processing routine at best. Gallilean addition of velocities is then modified by dividing the original result with a new result. A post-processing module.

In relativity, light still travels along straight lines but along a warped top

I'm not sure that you can say that it replaces the theory. You can use Newtonian mechanics exactly as Newton wrote them, without changing them one iota, but merely pass in the relativistic values rather than the classical ones. That doesn't sound like a replacement, that's a pre-processing routine at best. Gallilean addition of velocities is then modified by dividing the original result with a new result. A post-processing module.

In relativity, light still travels along straight lines but along a warped topology. The result is the appearance of light being bent by gravity, when it is space that is bent. Light is unaffected.

Where is the replacement?

Agreed and I will add that in 20 years or so we'll have new additions to Physics that augments Relativity and the exactness of what is actually going on with Force in a system will have a more complete picture, but we don't spit on the foundation of Newton and Einstein--we'll augment them.

That's a bit of an oversimplification of relativity. Some simple, highly idealized calculations can be done by just replacing an input with a modified version of it, or modifying the result of a Newtonian calculation. But, calculations involving significant gravity are far more complex. I would agree with you if we were talking about special relativity instead of general, or if the calculations were kept simple.

I thought the same when I read this piece of news yesterday. Journalists like to fill their sentences with words that sound appropriate to them: "[Newton's Law of] Gravity", "8.1 magnitude [in Richter's scale]"... and often they make mistakes.

I hope they took into account the possibility that they were exciting a mechanical resonance of the plate, which would cause it to vibrate and as a result occasionally be positioned differently and possibly intercept the neutrons at slightly higher or lower locations, corresponding to higher or lower energy.

The plate would have to be fairly wide and floppy to resonate at that frequency, but then it wouldn't have much gravitational attraction, so the plate is probably thick and chunky, and not very flexible. And it would have to be made of rubber to have any body resonance at that frequency.

Yes, we obviously need to be worried that the large team of scientists and engineers who designed and built this experiment have overlooked the most basic principles of freshman physics and mechanical design. Good thing we have the keen intelligence of Slashdot science critics to catch all these subtle flaws that would otherwise slip by the reviewers at Nature un-noticed. Should we also worry that the scientists are all part of the government conspiracy to cover up the true Time Cube four-side harmony perfection of gravity symmetry?

It doesn't talk about gravity either.
Although regarding neutrons, some people speculate that the Ark of the Covenant was radioactive since plagues of tumors followed it. [biblegateway.com] and people who looked into it quickly died. [biblegateway.com]

You see, cars are attracted to other cars and there are some grooves on the road so a car usually stays jammed into one of them. While there it is both morphed a corresponding Transformer (the transfomer depending of the groove and the car) and the car but if you look at it you may see a car or you may see a transformer ex: Megateron. I hope that this clarify any doubts you had about gravitational quantum transformers.

You may be able to wrap your head around the idea of the quantum states of an electron bound to a proton (that is, an electron in a hydrogen atom). The hydrogen atom is defined by having a particular potential energy that varies as a function of the distance from the proton. This is just like that, but instead of a potential energy that depends on the distance, r, from the proton like 1/r, we have a potential energy that varies as a function of the distance from the center of the earth, h, like m*g*h. Th

forgive me for this stupid question, but if neutrons have 0 charge, by what means does the upper plate attract them and the bottom plate repel them? Shouldn't the neutrons just ignore the presence of the plates and fall toward the center of the earth, aka down? or do we already have anti-gravity technology that i am not aware of

I didn't RTFA, but the "charge" for gravity is called "mass", which neutrons have, and the earth's gravity could be neutralized by setting up the plates vertically, so any movement of the neutrons towards the earth's center wouldn't coincide with a movement towards one of the plates. Oh, and both plates may attract the neutrons -- the summary only said that once the neutrons reached the plate surface, one of the plates would absorb the neutrons, and the other plate would reflect them.

According to the links, one plate is smooth, and the other is rough. so a neutron will glide over the smooth plate or be scattered at small angles but if it hits the rough plate it will be scattered more, on average. Why the difference? So that they have a different effect and you can tell if perturbing their course causes more to hit the smooth plate or the rough one.

The neutron's course can be perturbed by gravity. In the steady state, this means the neutron just drops in a parabolic arc following gravity, which at these length scales (microns) can be more determined by massive nearby objects (1/r^2 is huge) than by the distant center of the Earth (1/r^2 is tiny). (You might even get a setup where the top plate gravity is equal and opposite to the Earth's gravity, for objects that are close enough.)

Moving one plate nearer or farther away makes the arc change shape, changing how many neutrons are scattered for a given beam intensity and launch angle. Moving the plate in an oscillating motion at a given magnitude should give you an oscillating scattering measurement with a fairly constant magnitude. You would expect the number of neutrons scattered to be irrelevant of the frequency, when averaged over many cycles of the oscillation, if you considered gravity to be purely Newtonian (i.e., Newtonian gravity, f = GmM/r^2, is monotonic with changes in r, even when r is changing with time).

But they don't see that. They see distinct frequencies of plate oscillation that result in bumps or sharp bends in the average scattering.

It's a citation. Nobody said it has to be easy to get. If you think the article submitter is actually a shill for Nature trying to drum up funds by getting a bunch of Slashdotters to pay $18 for a copy of the article, well, you're a new kind of crazy I haven't seen before.

Spectroscopy is a method typically used to assess an unknown quantity of energy by means of a frequency measurement. In many problems, resonance techniques1, 2 enable high-precision measurements, but the observables have generally been restricted to electromagnetic interactions. Here we report the application of resonance spectroscopy to gravity. In contrast to previous resonance methods, the quantum mechanical transition is driven by an oscillating field that does not directly couple an electromagnetic charge or moment to an electromagnetic field. Instead, we observe transitions between gravitational quantum states when the wave packet of an ultra-cold neutron couples to the modulation of a hard surface as the driving force. The experiments have the potential to test the equivalence principle3 and Newton’s gravity law at the micrometre scale

Generally, a quantum mechanical system that is described by two states can be understood in analogy to a spin-1/2 system, where the time development is described by the Bloch equations, assuming two states of a fictitious spin in the multiplet, similar to spin-up and spin-down states. In magnetic resonance of a standard spin-1/2 system, the energy splitting results in the precession of the related magnetic moment in the magnetic field. Transitions between the two states are driven by a transverse magnetic radio frequency field. Similar concepts can be applied to any driven two-level system, for example in optical transitions with light fields. Variations are inherently connected to high-precision measurements such as atomic clocks6, atom interferometry7, nuclear magnetic resonance8, quantum metrology9 and the related spin-echo technique10. The sensitivity reached so far11 in the search for the electric dipole moment of the neutron is 6.8×1022eV, or one Bohr rotation every six days.

In this Letter, we demonstrate that energy eigenstates in the gravity potential of the earth can be probed using a new resonance-spectroscopy technique, using neutrons bounced off a horizontal mirror. This spectroscopy technique has in common the property that a quantum-system is coupled to an external resonator. Quantum mechanical transitions with a characteristic energy exchange between the coupling and the energy-levels are observed on resonance. A novelty of this work is the fact that the quantum mechanical transition is driven by an oscillating field that does not directly couple an electromagnetic charge or moment to an electromagnetic field. Instead, we observe energy transfer on resonance that is based on gravity-quantum states coupled to a modulator. We have named this technique gravity resonance spectroscopy, because the energy difference between these states has a one-to-one correspondence to the frequency of the modulator, in analogy to the nuclear magnetic resonance technique, where the energy splitting of a magnetic moment in an outer magnetic field is related to the frequency of a radio-frequency field. This is possible because of the feature of the quantum bouncing ball12, 13 that the levels are not equidistant in energy. The linear gravity potential leads to measured14, 15, 16 discrete non-equidistant energy eigenstates |nright fence. A combination of any two states can therefore be treated as a two-level system, as each transition can be addressed by its unique energy splitting or, in our case, by vibrating the mirror mechanically at the appropriate frequency. It has also been proposed to realize transitions between gravitational quantum states by means of oscillating magnetic gradient fields17. The physics behind these transitions is related to earlier studies of energy transfer where matter waves bounce off a vibrating mirror18, 19 or a time-dependent crystal20, 21. In the latter case the transitions are between continuum states, in the quantum bouncer the transitions are between discrete eigenstates. Optical dipole traps of atoms are reviewed in ref. 22.

Now I will freely admit that my education in Physics is abysmal (I got a D in my Physics A-levels:( ), but utilising the god^H^H^Hslashdot-given-right to talk on any topic without even RTFA, I ask a question:

The way that the lower/upper plates "repel/attract" the neutrons is not to be due to familiar forces (e.g. electromagnetic, gravitational, weak, strong), but rather due to quantum scattering effects from the bulk of nuclei in the plate material (which can be either attractive or repulsive, depending on material composition) based on the Fermi exclusion principle (identical fermions, such as neutrons, cannot occupy the same quantum state, resulting in effective forces between them not caused by any other for